What Does “20 Amp Hr Rate” Mean?

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Lead acid batteries are complicated things. Take some lead and sulfuric acid, stick them in a container together and you end up with a potential chemical reaction that can produce electricity. The question is, when you need to know how much electricity is able to be created with that chemical reaction, what is the standard rating, and how is it set? We have answered this question the complicated way on the Peukert's Law Tutorial, but here's a more basic look at determining what you need to know.

Several years ago when you would go to buy a car, the little tag on the window of a coupe might have read 42 mpg. Of course everyone knew that the only way to make that car get 42 miles to the gallon was to drive at 25 miles per hour on a newly paved level track. So in 2008 when the EPA updated their standards, the ratings all dropped to much more realistic numbers that most light-footed people can achieve. Battery ratings are very similar. The faster a battery is drained, the less overall amperage is available. The battery’s AH rating goes down the faster you use it. This is not the same thing as saying you use up what is available faster, but you actually decrease the total overall capacity itself.

To ensure that ratings are given in a realistic way, lead-acid batteries have a few parameters on how they get that “AH” rating. In order to get an AH rating, the battery that is being tested has to be drained down to 0 over the course of a specified amount of time. The amount of amperage that it took to get it down to zero, over that specified amount of time constitutes the AH rating.

Because of the Peukert effect (aka, the faster a battery is drained, the less overall amperage is available), if you discharge a battery over the course of 100 hours, the AH rating looks higher than if you discharge that same battery over the course of 1 hour. So, there has to be a standard.

For deep cycle batteries the standard rating is 20 hours. So, if a battery has a rating of 100AH @ 20Hr rate, then that battery was discharged over 20 hours with a 5 amp load. Starting batteries, on the other hand, are typically rated at 10Hr rate, because they are used faster, so the 20Hr rate is not as important. So, that weird 20Hr rate that you see after the AH rating on batteries tells you that the rating in question is the realistic, common rating—rather than an over-inflated number to make the battery look better than it really is.

BatteryStuff Tech We have an excellent Calculator | Sizing a 12 Volt Battery to a Load that can help you answer your question. It will depend on the chemistry of battery you want to buy. Once you input your figures into the calculator it will tell you what AH rating is needed based on the battery you select.

Lee How do I determine the proper rating for a battery to run a CPAP machine for 3 nights camping without recharging? and how do I determine how much of a load the CPAP machine is? I can buy the battery and test it at home, but don’t want to waste money on a battery which doesn’t have enough, or that has too much capacity.

BatteryStuff Tech You first will need to look to your CPAP machine to determine its voltage and amp rating. There are machines that are able to run directly off a 12v battery, but if yours is not you will most likely have to power through an inverter. If you have to go through an inverter you will first need use our Calculator | DC to AC amperage conversion run through an Inverter to determine the 12v amp draw off the battery. After you know your 12v amp draw you can size your battery using our Calculator | Sizing a 12 Volt Battery to a Load. In your case just put the total of hours you require the battery to last in order to run your machine for 3 days.

Lee It can run directly on 12V DC, so it doesn’t need an inverter. I have the correct DC cable from the manufacturer. I now have a new and fresh 12V 55A AGM battery, and an appropriate smart charger. I can’t seem to find any specs on how much power the machine draws – and there are variations depending on the pressure settings, and whether the humidifier is used anyway. I was going to fully charge the battery and just hook it up and see how much it draws off the battery for a night. Does that sound like a good plan? Thanks for your help. The info I’ve gotten from the tutorials has been extremely helpful.

Jim Ebay (or almost any hardware store) sells devices to check how much electricity is being used. Usually they go by names like “WattMinder”, “WattChecker” or something similar. You plug it into the wall, and then plug your lamp or CPAP machine, or whatever into it. It has a digital readout that says how many watts or amps it is using. You would be plugging your 110v to 12 volt converter (wall wart) into it so there would be some inaccuracy there, but it should give you a rough idea of how many watts your CPAP machine draws. Once you know that, you can mathematically figure out how many amp-hours you would need for your camping battery.

Another idea is to just get two batteries and leave one on a trickle charger in your car, and then swap it out with the second battery every day you are camping, that way you’ll always have a fresh battery. Of course if you are 20 miles from your car, that won’t work, but I’m sure you can work something out.

BatteryStuff Tech You could run it all night, or detect the amp draw while everything is hooked up and running. Most volt meters have have a setting to detect the amp draw, but often limit themselves to a 10 amp max. If you exceed the 10 amp rating you blow the internal fuse, and most of the time it isn’t replaceable.

David I have 2 20ah batteries to power a boat trolling motor. Currently I hook 1 up and when it drains I hook up the other. One is wet the other is lithium. Max amp draw on the motor is 30 amps. Q1- on 1 20ah battery, can the motor draw 30 amps? I’m wondering if the motor is pulling 30amp from the 20ah source. It would seem to me that it could, and last 45 minutes. It lasts much longer than that on full speed. So Q2 – if I put these in parallel would the motor
Run faster or would the parallel connection just make
The battery bank last longer?

BatteryStuff Tech Good question! You can apply a 30 amps load to a 20 AH rated battery, but it isn’t going to last as long as you would expect. The 20 AH rating the battery has is most likely rated over a 20 hour time frame. So that battery could handle a 1 amp draw for 20 hours, but when you put a large draw on the battery it will not last as predicted. If you want a in-depth explanation in regard this phenomenon, I would suggest our article on Peukert’s Law | A Nerd’s Attempt to Explain Battery Capacity. I compared an 18 AH rated battery we sell and entered its ratings into our Calculator | Determine Run Time for Specific Load, and that battery could only last 12 min given the 30 amp load before you start to excessivley discharge the battery. As far as your second question you would only see an increase in capacity, however it isn’t recommend to mix an match battery types.

James S. The 20 amp hour rate is for taking the battery down to 0%. Since we never want to discharge the battery below 50%, the first thing we need to do is divide the amp hour rate in half. For a 215 AH battery, that gives us 107.5 amps to play with. Take whatever you want your load to be and multiply it by the number of hours you want to run it. The total cannot exceed 107.5 amps. So lets say you have a 400 watt inverter that you want to run. 400watts / 12volts = 33.333 amps. And 107.5 amps (battery capacity) divided by 33.333 amps (power inverter load per hour) = 3.225 hours that you can power the 400 watt inverter. Now, a 400 watt inverter draws slightly more than 400 watts from the battery. But we can tell from the numbers that we should get 3 hours run time from the battery without exceeding 107.5 amps. My 50 watt inverter has a 5 amp draw, which is actually 60 watts instead of 50. So when I do the math I calculate at 5 amps. And 107.5 / 5 = 21.5 hours of run time. And because my battery is connected to solar panels, I know that I can run my 50 watt inverter 24 hours a day as long as my panels get at least 3 hrs of sunlight each day. I also have a 1000 watt inverter connected to my battery that I use to run a 700 watt microwave. Because I only use the microwave for 20 minutes or less each day, my solar panels are able to replace that power as well as replacing the power used by the 50 watt inverter which is never shutoff.

BatteryStuff Tech James, the 50% discharge level is a good rule of thumb, but because of Peukert’s constant the real run time could be a good bit shorter than the straight math would tell you. Another rule of thumb is that you can use the 50% discharge level without running the complicated math for up to 5% of the battery capacity. Meaning, for a 100AH battery (rated at a 20 hr rate) you can calculate the run time linearly for loads up to 5 amps. However, if your load for that 100 AH battery was going to be 10 amps or 15 amps, then you would want to use the calculator:
/kb/tools/calculator-for-load-specific-run-time.html

For more info:
/kb/tools/peukert-s-law-a-nerds-attempt-to-explain-battery-capacity.html